ATM telecommunications systems and method for routing narrow band traffic

ABSTRACT

A telecommunications system comprises an asynchronous transfer mode (ATM) network having uncommitted bandwidth, and a plurality of adaptive grooming routers (AGR) coupled to the network. The AGRs comprise a group adapted to function as a virtual transit exchange whose fabric and control are distributed over the group. The visual comprising the AGRs incorporates independent connection control and call routing functions and has means for determining the current system status whereby to set up narrow band connections across the ATM network based on that status determination.

RELATED APPLICATION

This application is a division of U.S. patent application Ser. No.11/055,787, filed Feb. 10, 2005, which is a reissue of U.S. Pat. No.6,519,257.

BACKGROUND OF THE INVENTION

This invention relates to digital communications systems and inparticular to systems embodying asynchronous transfer mode (ATM)technology.

The asynchronous transfer mode (ATM) technology is a flexible form oftransmission which allows any type of service traffic, voice, video ordata, to be multiplexed together on to a common means of transmission.In order for this to be realized, the service traffic must first beadapted typically into 53 byte cells comprising 5 byte headers and 48byte payloads such that the original traffic can be reconstituted at thefar end of an ATM network. This form of adaptation is performed in theATM adaptation layer (AAL).

As telecommunications networks increase in complexity and carryincreasing volumes of traffic, the current procedures for setting upconnections between subscribers are limiting the performance of thesenetworks. In particular, congestion may be caused by attempting toconnect to a subscriber who is already busy, or by attempting to choosea route through an already congested part of the network. Thus equipmentand resources can be wasted in attempts to set up calls which cannot becompleted. A further problem is that of scalability. As the networkexpands to accommodate increased traffic and a larger number ofsubscribers, there is an increasing need to facilitate integration ofnew equipment into an existing network without simply increasing thecongestion problem.

SUMMARY OF THE INVENTION

The object of the invention is to minimize or to overcome thesedisadvantages.

According to one aspect of the present invention there is provided adistributed telecommunications exchange system having independent callrouting and connection control for setting up connections across thesystem.

According to another aspect of the invention there is provided atelecommunications system, including an asynchronous transfer mode (ATM)network, and a plurality of adaptive grooming routers (AGR) coupled tothe network, wherein the AGRs comprise a group adapted to function as asingle distributed or virtual transit exchange whereby in use to set upnarrow band connections across the ATM network.

According to one aspect of the present invention there is provided adistributed telecommunications exchange system having means fordetermining the current status of the system whereby to effect routingof narrow band traffic across the system.

According to another aspect of the invention there is provided a methodof communicating resource availability to maintain performance of adistributed exchange system under overload conditions.

According to a further aspect of the invention there is provided amethod of routing telecommunications traffic in a system including anasynchronous transfer mode (ATM) network having uncommitted bandwidth,and a plurality of adaptive grooming routers (AGR) coupled to the ATMnetwork, which AGRs comprise a group adapted to function as a virtualtransit exchange whose fabric and control are distributed over thegroup, the method including determining the current system statuswhereby to set up narrow band connections across the ATM network basedon that status determination.

The technique provides for the separation of call routing and connectioncontrol together with the advertising of the system status. This ensuresa wide range of scalability so that the application of dynamic trunkingtechnology provides scalability in a traffic sense. Further, theseparation of call routing and connection control provides a distributedcomputing environment resource. Because the distributed exchange managesits own internal traffic, effectively it provides means for balancingthat traffic to the fabric and makes its own internal routing decisions.

Reference is here directed to our co-pending applications Nos.9410294.4, 9410295.1, 9411944.0 and 9502552.4 which relate toarrangements and methods for handling narrow band traffic in an ATMcommunications network.

BRIEF DESCRIPTION OF THE DRAWINGS

Embodiments of the invention will now be described with reference to theaccompanying drawings in which:

FIG. 1 illustrates an AGR network system that is the preferredembodiment of a large distributed exchange system;

FIG. 2 illustrates signaling separation in the network of FIG. 1:

FIG. 3 illustrates the provision of advanced services in the network ofFIG. 1;

FIG. 4 illustrates the principle of dual homing and diverse path routingexternal narrowband exchanges to the distributed exchange system;

FIG. 5 illustrates the connection of the large distributed exchange to ahierarchy of narrowband exchanges;

FIG. 6 illustrates an overview of handling a connection set-up requestand the roles of call routing, connection control and voice-pathconnection termed wormholes;

FIG. 7 illustrates the principle of using existing N-ISUP signaling asthe means for routing calls across the distributed exchange system atthe Call Routing layer;

FIG. 8 illustrates the principle of controlling routing and congestionin the Call Routing and Connection Control layers;

FIG. 9 shows a AGR protocol stack for the network of FIG. 1;

FIG. 10 illustrates a means for specifying a speech path across thedistributed exchange system;

FIG. 11 illustrates the functional topology of the physicallydistributed exchange system;

FIG. 12 illustrates an arrangement for determining available trunkcapacity on a Virtual Trunk Route (VTR);

FIG. 13 illustrates an arrangement for determining a Virtual TrunkGroup's (VTG) capacity;

FIG. 14 illustrates an arrangement for advertising congestion on aremote node's VTG in the distributed exchange system;

FIG. 15 illustrates an arrangement for advertising congestion on aremote node's VTR in the distributed exchange system;

FIG. 16 illustrates an extension of the arrangement of FIG. 13 forspecifying available voice band processor capacity;

FIG. 17 illustrates a preferred signaling process to create a speechpath between AVJs in the network of FIG. 1;

FIG. 18 illustrates the information content required to establish orremove a narrowband connection;

FIG. 19 illustrates a method for tracing a narrowband connection throughthe large distributed exchange from a trunk circuit; and

FIG. 20 illustrates the overall protocol for establishing a connectionend to end in the layers Call Routing and Connection Control and atphysical layers;

DESCRIPTION OF PREFERRED EMBODIMENT

Referring first to FIG. 1, this depicts the functional architecture ofthe Adaptive Grooming Router (AGR) network system. The AGR works on theprinciple of a single fabric switch (SFS) incorporating a novel devicean adaptive virtual junctor (AVJ), that adapts narrowband traffic to andfrom the ATM adaptation layer e.g. (AAL-1), and also provides the meansto time switch narrowband channels. Using a standard ATM switchingfabric, the AVJ allows a fully scalable narrowband exchange to be built,by communicating through the switch fabric using a fully interconnectedmesh of dynamically sized trunk groups or virtual channel junctors(VCJ), which gives a non-blocking space switching performance. The AGRextends the scale of this principle to the wide area by usingdynamically sized Virtual Trunk Groups VTGs to construct a fullyinterconnected mesh of AGRs that can dynamically adapt to thecommunity-of-interest in the wide area network. The network of AGRs istermed an AGR network system, and functions as a large distributednarrowband exchange, that at a minimum can replace a single transitexchange, and at a maximum replace the existing trunk/transit layers ofan hierarchical narrowband exchange network. The term Adaptive GroomingRouter (AGR) is used since the node grooms traffic to/from localexchanges according to instantaneous demand for a trunk/transitconnection. The VCJs and VTGs function on the principle of dynamicstructured data transfer (DSDT) to provide a end-to-end narrowbandconnection capability, and the ability to perform path tracing.

The capacity of a single AGR node is dependent on the size of the hostATM switch, the number of connected AVJ devices and the capacity of theAVJ device itself. Further, the AVJ device can be dedicated to performeither a trunking function (termination of synchronous narrowbandcircuits) or a grooming function (generation of traffic groups fortransmission across the WAN) equivalent to a tandem switching functionof NB networks. Therefore both the overall capacity of the node and themix of traffic (intra and inter node) can be dimensioned according toneed. For example, consider a node built using 8,000 channel AVJdevices, hosted to a 10 Gb/s ATM switch. A total of 11 fullyinterconnected AVJ devices with four dedicated to trunking and 7 togrooming yields node with a total of 70,000 circuits (32,000 TDM trunksand 28,000 WAN trunks). In this configuration the node can terminate upto 28,000 Erlangs of traffic from the synchronous local exchange networkwith over 80% of this traffic capable of being carried across the WAN.If however, the amount of WAN traffic is less, then one or more of thegrooming AVJ devices could be sacrificed for a further trunking AVJ thusincreasing the amount of synchronous trunking capacity.

For AGR network resilience, a minimum of two VTGs will interconnect allof the nodes within a network. The capacity of the routes can bedimensioned according to the communication demand—high capacity VTGsconnecting high traffic routes, 47 trunks and up and low capacity VTGsconnecting low demand routes 6-46 trunks. In this manner the overallgrooming capacity of a node can be flexibly dimensioned to connect toeither a very large number of nodes (with modest traffic on each route)or to a lower number of nodes (but with a large amount of inter-nodaltraffic or a mix of both. As an example, consider the node describedabove, with 28,000 inter-nodal trunks. In one configuration this couldsupport 178 similar nodes each node interconnected by two VTGs with anaverage capacity of 80 trunks (the capacities of the individual VTGs canof course dynamically increase and decrease according to demand). Inthis manner a network capacity of over 5,000,000 narrowband trunks canbe supported with over 80% of this traffic able to traverse the WAN atany one time.

FIG. 1 also shows the manner in which the AGR node provides e.g. N-ISDNSS7 signaling capability and call routing.

The arrangement provides a means for establishing, maintaining andremoving connections in an AGR network. The AGR network system iscontrolled from a proxy call routing server in an ATM network to providenew services to users in the network, stemming directly from theaforementioned separation of call routing from connection control whichis illustrated in FIG. 2 which illustrates the manner in which adaptingsignaling physical layers to ATM allows evolution from STP basedsignaling to signaling as an ATM network service.

FIG. 3 illustrates the principle of an AGRNS that provides a connectionengine, controlling narrowband connections between endpoints specifiedby the local Call Routing subsystem, or by a proxy server. Therefore theproxy server may implement signaling systems that go beyond thecapabilities of the N-ISDN SS7 signaling subsystem in the AGR node, andprovide for example multimedia services and control thepoint-to-multipoint hardware capability of the AGR. In the exampleillustrated, the D channels of ISDN Primary Rate Access are routed to aframe processor located in the ATM network, accessed by a VirtualChannel Connection. The frame processor takes HDLC frames and adaptsthem to/from AAL-5. The D channels may carry HDLC frames that areadapted to the broadband MTP-2 protocol Signaling ATM Adaptation Layer(SAAL) and then routed to the proxy server. The HDLC frames may containany layer 3 or higher layer of the OSI model as signaling messages fordirect interpretation by the software within the proxy server. Thesemessages are translated into commands that use the ApplicationsInterface of the connection control of the AGR node or are formed intoSS7 messages or any other suitable proxy signaling protocol that can beinterpreted by the Connection Control layer of the AGR software toinstruct it to control narrowband connections between specifiedendpoints. By separating the service aspect from the connection aspectin this manner, new services may be made available in the network.

Referring now to FIG. 4, this illustrates the principle of dual homingof narrowband exchanges to the AGR network system. In hierarchicalnarrowband transit networks, a local exchange in the lowest tier mustprioritize its traffic across many small routes depending ongeographical destination of a call and traffic congestion levels. Sincethe AGR network system performs a grooming and dynamic handling oftraffic demand, the local exchange can now simply “hook on” to the AGRnetwork system using a large voice route. The large voice route hasincreased Erlang efficiency, and providing two large voice routesprovides a degree of resilience if the traffic is balanced across thetwo routes. The AGR network system can also make use of the dual homingfacility to balance traffic internally, since there is a choice ofdestination AGR when communicating with any dual homed narrowband localexchange.

FIG. 5 illustrates the connection of the AGRNS into an existingnarrowband network hierarchy of exchanges. In such a narrowband networkit is traditional to provision the network to have a choice of routeswith priority given to the most direct route between exchanges. Howeverthe routes will be sized such that they experience a differing degree ofblocking probability that decreases with decreasing priority, such thata backbone route via exchanges in the highest tier has a much lowerblocking probability than routes directly between local exchanges. TheAGRNS may be deployed to replace just one such exchange in tiers 2 or 3,or to provide a backbone narrowband network using ATM transport thatcould replace part or all of the narrowband transport and exchanges.This would ultimately form a single tier transit network. Consequently,the AGRNS must in general be able to cope with the prioritization ofvoice routes that connect it to the narrowband transit hierarchy. Thesevoice routes would suffer congestion because of occupancy and alsoexchange processor activity, as described in the ITU White BookStandards on SS7. Internally the AGRNS must cope with the sharedutilization of the ATM transport and with the blocking caused byfragmenting its grooming capacity across a number of physical devices.The load balancing arrangement shown in FIG. 5 exploits the dual ormulti-homed connectivity in the narrowband network to even out trafficoccupancy on external voice routes and internal trunk groups so as delaythe onset of congestion and maintain carried traffic levels underoverload conditions by rejecting offered traffic at the periphery of theAGRNS.

Reference is now made FIGS. 6 to 20 which illustrate stages of theconnection set up in the network of FIG. 1 in response to a set uprequest. The diagram of FIG. 6 shows two separate functions, callrouting and connection control. FIG. 6 also shows the stages ofprogression of the call. The incoming initial address message (IAM),which may comprise the initial address message of the number 7 signalingsystem, goes to call routing which makes decisions based on a digitanalysis by using as many digits as it needs to determine the outgoingvoice route or set of voice routes in the narrowband network (see FIG. 2b) and it may, if necessary, wait for other signaling messages. Oncecall routing has chosen the candidate set of voice routes it can thenmake decisions based on static prioritization of those voice routes.I.e. a voice route through a local exchange can be used in preference toone through a transit exchange, which preferably is used less often. Thecall routing then makes an assessment of congestion on those voiceroutes. This could either be a total circuit occupancy of the voiceroute or it could be congestion associated with an exchange and theprocessing power it has available to handle signaling messages, e.g. inaccordance with ITU standards on SS7. It will be appreciated thatcongestion can arise both from excess voice route utilization which isusing up circuits in those voice routes and from exchange loading whichis a congestion indication which is exchange specific but notnecessarily associated with the voice route which is to go between theAGR network system and the exchange.

Connection control is responsible for ensuring that a connection can bemade between the end points P and Q through the AGR network system.Messages can be passed from call routing (CR) to connection control (CC)which can then eliminate routes with respect to internal congestion, ormay be continually passed from connection control to CR such that callrouting may eliminate candidate routes in priority order. Internalcongestion can be due to having to share the ATM network with othertypes of traffic so that voice traffic demands have to compete withother demands for the same resources, and also because inherently in thedesign of the AGR there is a fragmentation of resources in the physicaldevices. Whilst there may be a certain amount of capacity available on apooled resource, it may in fact be fragmented over a number of physicaldevices and therefore steps should be taken to balance the load betweenthe physical devices. It is possible to arrive at a situation where thecarrying capacity is much less than the design capacity because thiscapacity cannot be accessed fully as a result of fragmentation. In suchcircumstances connection control eliminates the corresponding route andthen passes the message back to call routing for forwarding to adestination AGR.

Thus, the destination AGR choice is based on the highest priorityaccessible, reachable, voice route which immediately identifies the AGRfrom which that voice route emanates so from that point on signalingmessages are forwarded to the outgoing (OG) call routing on the otherside of the network. At that stage the outgoing Call Routing isresponsible for determining whether it can make a particular trunkcircuit selection. A voice route consists of many trunk circuits, socall routing has to deal with the possible race conditions of incomingsignaling on that side which may want to select the same voice circuit,because they are both way circuits, from that same voice route. Theremay be various circuit selection policies. Call routing monitors theprogress of those signals and will then perform the final selection ofthe trunk circuit in that given voice route with respect to whetherthere is a race condition, which is termed glare, and also taking intoaccount whether certain circuits are out of service, not provisioned, orthe type of call, e.g. whether it is a test call. Once the circuit hasbeen selected, call routing passes this information to connectioncontrol and begins the third stage of the process to actually connectthe voice path between the end points P and Q, either in aunidirectional or bidirectional manner in the network system, this beingreferred to as a worm which is a specification of the nodes that thevoice path will pass through. The content of the worm specifies theconnection of the circuit at each stage of the connection in turn fromtrunking AVJ to grooming AVJ and back from grooming AVJ to trunking AVJin the other remote AGR. Essentially the mechanism is driven by the wormbeing passed forward in a chain and then passed back as a form ofacknowledgement. Once it has reached the outgoing stage again then thetrunking AVJ responsible passes it to connection control which passesout a message to Call Routing which can then send out the outgoing IAMsignal. This illustration of the worm is in fact a main path which showsthe whole passage of a call set up. The call handling that isencompassed on the AGR has taken into account external and internalcongestion. When the call routing is a proxy call routing, i.e. the AGRwere just providing a connection entity, it is connection control'sresponsibility to handle internal congestion, and the particular form ofcall routing or special services would take into account their own meansof determining external congestion, for the signaling scheme in use, andthe handshake between Call Routing and connection control and theselection of the outgoing voice route, applies equally to these proxyservers.

FIG. 7 shows how the existing SS7 signaling scheme can be used to signalmessages to and from the AGR network system. In FIG. 7, local exchangeLEA is a local exchange but it is also providing a signaling transferpoint for local exchange LEB, so that local exchange LEB signals itsmessages through local exchange LEA. The AGR network system consists ofAGRs 41 to 44. one of which (41) is the incoming AGR.

Outgoing AGRs are illustrated just to illustrate a means for balancingtraffic load to local exchange LEC which happens to be dual homed ontothese two outgoing AGRs. The local exchange LEA can select a destinationpoint code. The LEA when it has performed digit analysis on the callderives a destination point codes (DPC) and from the destination pointcode derives particular signaling route and then a particular signalinglink set and a particular signaling link to reach the AGR system layerMTP-3 (Message Transfer Part 3) whereby to determine whether themessages are indeed for this AGR, which from the DPC determines whetherthe messages are being forwarded to another exchange, either in the AGRsystem, or external to the AGR system, or whether they have in fact beenrouted incorrectly. From the destination point code and the originatingpoint code, the MPT-3 can tell whether to handle this messageinternally, which is passed to the narrow band ISDN User Part (NIS UP),and determines whether the message came from local exchange LEB or fromlocal exchange LEA, in which case it may be handled accordingly, orwhether the destination point code differs from the AGR in this case,LEC, in which case there is an STP function whereby messages areforwarded to MTP3 in one of the outgoing AGRs using SAAL which is thebroadband signaling ATM adaptation layer. If the destination point codeand originating point code imply a signaling route, the message ispassed to N-ISUP, which from the destination point code and theoriginating point code associates that signaling route with a particularincoming voice route, and the CR entity that takes the signalingmessages and, by digit analysis, determines the possible set ofdestination point codes, i.e. the point code implies a possible set ofoutgoing voice routes that connect between the AGR network system anddestination NB exchanges which therefore implies a possible set ofoutgoing AGRs which head-up those voice routes. Once a choice of voiceroute has been made with due consideration to load balancing, internalcongestion, to determine which is the outgoing AGR, a similar schemefrom Call Routing down through N-ISUP down to MTP3 signals the outgoingexchange, for example the local exchange LEC. Thus, the destinationpoint code of the AGR is now replaced with the destination point code ofexchange LEC, the originating point code is now replaced with that ofthe AGR network system rather than exchange LEA or LEB. The destinationpoint code will imply a particular voice route between that originatingpoint code and the new destination point code, local exchange LEC whichwould imply a particular signaling route and therefore a particularsignaling link set.

Referring now to FIG. 8, this illustrates the way in which call routingpriorities voice routes statically or dynamically according to networkcongestion statistics. Statically means a static prioritization of routeaccording to the three tier network where there is a first priority, asecond priority and so on, or joint first priority, and dynamicallymeans the use of more advanced algorithms such as dynamic call routingalgorithms.

Call routing passes the set of potential destination AGRs to ConnectionControl. Connection Control is aware of the topology of the AGR networksystem, which AGRs are connected together by virtual trunk groups. Theremay be more than one virtual trunk group (VTG) between any given pair ofAGRs. Therefore there is a choice for load balancing between VTGs, butcombined with the candidate set of AGRs there could be more than one VTGto choose from overall. Connection control maps the set of AGRs onto theset of candidate VTGs which are carried in virtual paths in the ATMsense, referred to as virtual trunk routes, and connection control canestablish from the set of VTGs those voice routes which are notreachable due to internal congestion. By knowing the capacity availableon the VTGs, because these are dynamically sized, connection controlknows from congestion criteria whether it can dilate any particular VTGand what the greatest likelihood of success will be for any givenchoice. This provides a facility to block routes which are unreachablein the network at the outset. By rejecting calls with as small amount ofprocessing as possible then, when the system is heavily overloaded thereis reduced processing and signaling generated for calls which can notcurrently be established across the network. By providing thisessentially negative feedback scheme, a call can be rejected at theperiphery of the AGR network before an abortive attempt to route thecall across the network and thus a high degree of carried traffic can bemaintained. Connection control does not necessarily know theprioritization of voice routes, but it will eliminate those voice routeswhich are not reachable without actually changing the priority andtherefore it can achieve a load balancing capability with voice routeswhich are of equal priority or may have changed their dynamic priorityfrom the call routing perspective. Call routing can thus select onegiven route which has a high degree or certainty of success andtherefore need be only indirectly aware of the internal resourceavailability.

A preferred AGR connection protocol stack is illustrated by way ofexample in FIG. 9.

FIG. 10 shows the final stage of the path set up process at which,having selected the outgoing circuit, with knowledge of the incomingcircuit, the voice path is now to be set up therebetween. Because theAGRs have fixed topology, they have an inter-AGR stage which is shown aszone B in this figure. This is the VTG the virtual trunk group andintra-AGR which is the Virtual Channel Junctor VCJ, which is for mostpurposes identical to the VTG and works on substantially identicalprinciples. In this ATM based domain it is necessary to connect from thetrunking AVJs, which interface with trunk circuits directly tointermediate stages which are called grooming AVJs because they groomtraffic from and to the wide area (the inter-AGR traffic) and finally tothe trunking AVJ which is associated with circuit P. The worm thuscontains five identifying features which are illustrated here as being,the outgoing circuit Q, the next stage which identifies a grooming AVJthat will connect us to our chosen VTG, which therefore implies aparticular VCJ, the VTG chosen by Connection Control and the VCJ whichwill take us to the trunking AVJ which interfaces directly with circuitP. By passing this worm forward from controllers built into the AVJ,they can each set up their connection maps to ensure that there is avoice path continuously from circuit P to circuit Q in either aunidirectional or a bidirectional scheme and that could form a forwardor a reverse voice path. Passing the worm to and fro to one end and thenback is a ready form of acknowledgement of the signaling scheme.

FIG. 11 illustrates the functional topology of the system and shows amethod for achieving both load balancing through a number differentroutes between AGRs and also for providing physically diverse routes forresilience purposes. Each AGR consists of trunking AVJs, and groomingAVJs which interface to a wide area network which has an ATM switch.This is a non-blocking ATM switch described earlier. Any local trafficin an AGR would simply be passed between its trunking AVJs. Any widearea network would be passed to a grooming AVJ for grooming on to a VTG.To achieve physically diverse paths one must ensure that those pathspass through a minimum of at least two switches in the ATM network.These switches are the two VC cross-connects shown and since a VTG is avirtual channel connection, it can be cross-connected at the VC leveland by ensuring the ability to connect between any pair of AGRs bypassing through at least two VC crossconnects in the network ensures aphysically diverse path because the crossconnects must be distinct, andtherefore use distinct ports, and therefore there is no single point offailure on those routes. In the path between an AGR and a cross-connectthere is a virtual trunk route (VTR) which in ATM terms is a virtualpath. A VTR can pass through any number of intermediate switches in theATM network functioning as virtual path cross-connects which have notbeen shown for clarity, but logically it is a path from AGR to VCcrossconnect and whatever bandwidth is associated with the virtual trunkroute as it emanates from the AGR is identical to the capacity it hasarrives at the virtual crossconnect. Thus each VTG and its potentiallyphysically diverse pair is contained in at most two VTRs between any twoAGRs. Thus an AGR can determine in its local section of VTR1 to the lefthand side, VTR2 to the right hand side, what capacity has beennegotiated with the ATM network, and what the available bandwidth iscurrently for calls in progress. VTGs are carried within virtual trunkroutes and a virtual trunk route can comprise a number of VTGs, whichare segregated at and multiplexed at the VC cross-connect, hence theutilization of VTR1 is not necessarily the capacity of VTR2, there beingno necessary association. AGR.1 is aware of the capacity and utilizationof VTR1 on the left hand side, and the AGR2 on the right hand side knowsthe capacity and utilization VTR2 then the end to end capacity andutilization is known through the ATM network, whatever the number ofintermediate switching stages, and there is no isolated section whosecapacity and utilization could not be determined in this manner, hencethe AGRNs can determine the congestion level in the network.

Referring now to FIG. 12, this shows a method of determining the sparecapacity on a virtual trunk route. The virtual trunk route statisticsare processed by a VTR capacity monitor which retains the maximum VTRsize which is the bandwidth/capacity negotiated with the ATM network,and the maximum bandwidth that one could use on the route. Some of thisbandwidth may be reserved for maintenance purposes, but the rest isdevoted to VTG bandwidth. The VTR CU also has information on the VTGsthat the VTR comprises and, by subtracting the current absolute sizes ofthe VTGs from the maximum usable bandwidth of the VTR, results in thespare capacity of the VTR, and whether that route is be congested ornot. The VTR CU is ideally located in the Connection Control layer ofthe AGR control system.

A VTR is a shared resource in the Connection Control, and thereforeinterrogation for its spare capacity can be single threaded on a call bycall basis, providing a ready means to share out the spare capacity. TheVTR CU information could be updated every time a call is modified in aVTG; it comprises as a means to reduce signaling, one could have a fixedtime interval, for update, between which the VTR CU grants a credit perinterval to each VTG it comprises based on that VTG's absolute—andchange in utilization in the previous interval, or any other appropriatescheme is applicable, so in this manner the VTR capacity monitor neednot be precise in operation. When the VTR capacity monitor has crossed acertain predefined threshold, and there may be several such thresholds,it flags processed calls a corresponding degree of congestion, andthresholds would be determined according to a desired networkperformance and efficiency. Bandwidth for VTRs may be decided accordingto desired network performance and those thresholds can be used as anindication of internal congestion in place of or in conjunction with theabsolute spare capacity, the indication of having crossed the thresholdmay provide a direct means of comparison and suitability betweencandidate VTRs in routing a call, and as a means for data/signalingcompression.

FIG. 13 illustrates the provision of grooming capacity information inthe AVJ device. Whilst there may be spare capacity on a VTR, the VTGs itcomprises could be controlled by distinct physical devices, i.e.grooming AVJs. Consequently whilst there may be spare capacity in theVTR there may no longer be any capacity on a grooming AVJ that controlsa VTG of interest, i.e. a VTG that is candidate for end-to-end routing.A grooming AVJ can signal call-by-call or, on occasion, the currentsizes of the VTGs that it operates to the respective VTR CUs, such thatthey know the size of VTGs, but a grooming AVJ can also send its sparegrooming capacity resource, and associate that spare resource with therelevant and respective VTGs to which it may apply in unequal measuredue to implementation compromises. Thus, the VTR capacity monitor cancheck the quality of a candidate route to VTG resolution, whether it hascrossed a threshold of congestion or whether any of the VTGs havecrossed a threshold of congestion by lack of capacity on their groomingAVJ. VTGs may include padding to ensure constant end-to-end delay in thenetwork. When some or all the channels in a given VTG are not used, byspecifying some minimum number of channels in a VTG, one ensures maximumend-to-end delay, and if those channels are not used then they arepadded, but because they are padded, adding a channel to the VTG wouldnot in fact increase the VTG size, so that capacity extra to the sparegrooming AVJ capacity. Under those situations the AVJ grooming sparecapacity can be ignored. So now we have a complete picture in the VTR'scapacity monitor of any local congestion.

FIG. 14 illustrates the process of advertising local conditions to thefar end through the VTGs, i.e. the AGRs to which a local AGR isconnected. Advertising local congestion in terms of crossed thresholdsor obsolete utilization can be done on a relatively infrequent basis andit provides the other end with information of the remote VTR sectioncongestion, because being able to dilate and increase a channel in a VTGat the local end is necessary but not a sufficient means to ensure thereis capacity at a remote end. This is a preferred means to ensure to avery high probability that calls will be connected on first attempt bymaking at the outset a proper assessment of whether a particular AGR isreachable by any given path and therefore reject blocked traffic atsource and hence the advantage of no impairment to the amount of carriedtraffic under overload conditions local or remote or universal.

A VTR capacity monitor can advertise its local congestion status interms of thresholds or absolute capacity to all or select remote VTRcapacity monitors to which it is connected, by VTGs through the VTGs itcomprises, by using for example the ATM F5 cell method or any otherequivalent signaling scheme which can be associated with a virtualchannel connection or VTG. A VTRCU can advertise to all those VTRCUs orto a selection, based on any selection criteria, for examplegeographical distance or logical hierarchical placement. The VTGs couldbe given for example a geographic location or, reach or distanceindicator, and thereby the congestion could be sent out as an indicationto only a local area if the distance is below a certain threshold withan associated congestion threshold, thereby only AGRs within a givengeographic locale will get signaled first and as congestion increasesthen it could spread to the wider AGR network system. One can envisageany scheme of selection criteria for any advantageous purpose, which maybe select or universal in application. A VTR capacity monitor obtainsremote VTR congestion indication for any VTG that it comprises and withregard to FIG. 10 a, congestion indications of remote grooming capacityby identical means and criteria from a remote VTR CU or remote groomingAVJ, for any VTG it comprises, and it can supply Connection Control anoverall preference by applying any type of weighted cost function to thelocal and remote congestion indicators and Connection Control can decidethereby the best routing policy with relatively up-to-date localinformation and relatively out-of-date advertised remote informationprior to making any call connection, to select a reachable voice routeand then forwarding the signaling remote AGR where its ConnectionControl may re-examine the best VTG route from the remote perspective,where it will have an up-to-date picture of its own congestion and arelatively up-to-date picture of the emanating AGR at the local end anda more up-to-date picture at the point when the final selection of VTGis made to send the worm back across the network to complete the voicepath connection.

As shown in FIG. 15, the scheme described above with reference to FIG.14 can be extended to the provision of voice band processors in anygiven connection. An example of this would be an echo canceller or halfecho canceller, where the exchange in its call routing from thesignaling will decide, because of the length of the route, or otherrelevant criteria on a need to apply echo cancellation for examplebecause the call has or will cross some certain delay budget. Because agrooming AVJ has a natural loop back capability where it simply isgrooming aggregated local traffic for the wide area network, there istraffic between its ATM domain and its TDM domain and then being mappedback into the ATM domain almost immediately, in performing the groomingfunction. This traffic may be diverted most advantageously through avoice band processor, for example a half echo canceller, connecting tothe grooming AVJs, this being the preferred embodiment although allother placements of echo cancellers or voice bands in the end to endconnection may be applied in the ATM or TDM domain. The capacity of thehalf echo cancellers, because they operate by a paired user port andnetwork port, also represent a resource depleted by utilization. Thegrooming AVJ is aware of and controls the maximum capacity of the halfecho canceller to which it is immediately attached, and duringconfiguration is informed which timeslots the echo canceller isconnected to. Because the grooming AVJ knows the identity of thosetimeslots and whether they have a half echo canceller configured forthem, it can automatically determine what the half echo cancellercapacity is that is connected to it, and it can report this togetherwith its own capacity to the VTR capacity monitor. In the United States,inter-exchange carriers on some of their long-routes always need halfecho cancellers, but they have a statistical distribution of the needfor half echo cancellers. One example would be 30% of all routes havinga need for half echo cancellers. By statistically distributing half echocanceller capability across all the grooming AVJs serving such routes,whatever grooming AVJ is chosen there is a statistical chance of havinghalf echo canceller capability if it is necessary. This has theadvantage over the use of dedicated grooming AVJs for half echocancellers by providing greater utilization efficiency and avoidance ofresonance depletion through fragmentation. Connection control may selecta VTG without congestion and satisfy the criteria of echo cancellerresource if applicable. The half echo cancellers can be readily enabledand disabled by simple loop back control within the grooming AVJ whichis another means of inclusion or exclusion of the half echo canceller inthe path and this is the preferred means of accommodating signalingschemes which introduce echo cancellers at the outset of a call and thenexclude them subsequently through subsequent signaling to ensure optimalplacement closest to called party. The grooming AVJs can optimize thisfunction by finding other timeslot means to loop back which do notutilize the half echo canceller resources. The VTR capacity monitor isthus a single reference for all the resources necessary to connect aparticular voice path through the network system.

Referring now to FIGS. 17 to 20, these show in sequence the means foractually setting up the voice path between trunk circuits P and Q. Thisincorporates a 5 part protocol for signaling over to a cascade of anarbitrary number of multiple stages, Each stage comprises an ingress anda egress process defined with respect to the ATM domain, egress beingtraffic that emanates from the ATM domain into the TDM domain in allcases. The egress process keeps a record of free channel timeslots andtherefore dictates where the offset in any given trunk group, VTG or VCJof a new channel is going to be by determining where a free timeslot isin relation to the other active channels already in the trunk group andtherefore the offset into that group when the new channel is added. Asimilar process applies to the criteria for removal of an existingchannel from the trunk group. Using the offset the egress process cansignal to the ingress process which is responsible for assembly of thattrunk group into ATM cells, and the ingress process is responsible foradding the channel through the dynamic structure data transfer functionof our earlier patent, and the egress process can detect the occurrenceof changes in trunk group size and therefore knows that the channel hasbeen connected and can receive subsequently further changes on thatparticular VTG or VCJ. This operation process, firstly is singlethreaded by VTG or VCJ, and the figure illustrates the two parts of theprocess, firstly signaling an offset from egress to ingress, andsecondly making the change to trunk group size, and forwarding the nextstage's offset as a fully pipelined process in a unidirectional manner,in which, connecting a voice path in the upstream direction from P to Qis achieved by signaling in the reverse direction from Q to P which is apreferred embodiment, because it minimizes the signaling transactionsinvolved, but does not imply the exclusion of signaling in the samedirection as the voice circuit and moreover the unidirectional mechanismmay be used to effect a bidirectional mechanism by signaling first fromQ to P and then in an identical manner for the reverse downstream voicepath signaling from P to Q. Once an egress process has signaled aningress process, the ingress process can transfer signaling to theegress process in the same AVJ for example, the grooming AVJ at stage 2which in turn determines the offset it needs for the VTG between stage 3and 2 and signals to that ingress process in stage 3 and so on throughthe system. The offset may be embodied in a cell referred to as a wormwhich passes to and fro to set up a bidirectional voice path. Provisionis made for AVJ stages 2, 3 and 4 to reject the connection because thecongestion indication given by the VTR capacity monitor could be out ofdate with respect to the true capacity status in any AVJ because ofdelay race for—resource conditions in the AGR network system. Therefore,a backtracking capability is built into the AVJ connection entities andprovision is made for the means to function correctly in the absence ofan advertising mechanism in part or in total. Provision is also made forcorrect operation in the absence of a VTR capacity monitor by abacktrack capability or similar recovery means. The AVJs at stages 1 and2, may optionally have capability to send a digital signature therebyguarding against any false simulation of that identity intentionally orotherwise. This provides a means of authentication such that eachtrunking AVJ at the end of a communication path knows that it hasconnected the voice path through to AGR network correctly providinganalogous integrity checks in existing narrowband exchanges which have,for example, a additional integrity pattern or other signaling schemesto ensure proper cross-connection of narrow band channels through thefabric. We envisage sending a digital signature for unambiguouslyidentifying the identities of the trunking AVJs at each end, and thecircuits P and Q prior to or subsequent to completion of the voice path,in the bandwidth of that voice path on erection, or prior to dissolutionof that voice path in order to provide a means to ensure continualintegrity checking on operations pertaining to network connectivity. Thegrooming AVJs are free to choose any free time slot in their connectionmap. This need not be prescribed by the worm, but the worm can beannotated with that information so that connection control software canreadily use the returned worm acknowledgement to check that the AVJdevices are performing the intended function. This is not a strictrequirement, there still being a means for disconnection of any trunkcircuit Q from any trunk circuit P by a voice path trace operation.

FIG. 18 shows an example of the main contents of the worm. At the top ofthe diagram are illustrated functional stages in which TID meansTrunking Identity, GID means Grooming Identity and the suffix denotesthe local and remote ends. The remote end AGR is the incoming end of thecall, and the local end is the outgoing end of the call since the wormbegins at the outgoing end. The incoming half echo canceller and theoutgoing half echo canceller, are positioned relative to the respectiveends of the call. The incoming side is the left hand side, the remoteside, because call routing forwarded messages to the right hand side andthe preferred embodiment is to send information in the backwardsdirection initially to minimize transactions. The items in heavy typeare those items which are necessary to connect a unique path through thesystem It should be noted here that the remote trunking time slot P andthe remote trunking AVJ identity can be forwarded from the remote end aspart of a call routing signaling message and no other topologyinformation regarding the remote AGR is required. For example, underfault or maintenance a remote AGR could independently change itsconfiguration, and the information forwarded need only be valid on acall-by-call basis. There is a window of opportunity of call-by-callresolution for making configuration changes without affecting any othercalls in progress in the system. The trunking AVJ ID at the local end,the trunking time slot and the grooming identity which heads theselected VTG are all locally stored topological information. Theinclusion or exclusion of echo cancellers is a call-by-call decisionprocess. The central part of FIG. 18 shows that in the AGR networksystem which connects to peripheral exchanges, any signaling forinclusion or exclusion of incoming or outgoing echo cancellers isassociated with the respective side of the call, the exchange on thelocal side only affects incoming echo cancellers. The exchange on theremote side can be arranged to signal only for outgoing half echocancellers and this avoids the need to store any information orconfiguration supervision of echo cancellers in the remote AGR. andlocal information need be retained during the lifetime and dissolutionof a call. The lower portion of FIG. 18 shows three main types of wormoperation namely, the connect, the disconnect and the modify commands.The connect command is for setting up the initial voice path and theitems which are grayed out are those which can be annotated by thegrooming AVJs and stored by connection control if necessary, but whichare not absolutely necessary for the system operation. In total thesecomprise an indication of inclusion or exclusion half echo cancellersand a specification of the exact route that the voice path should take.This facilitates the setting up of point to multipoint connections inthe AGR network system. The disconnect command can optionally includethe local information which is circled in the figure. In apoint-to-multipoint connection, then this information would be needed tospecify unambiguously the grooming AVJ identity and there may be morethan one voice path through that same grooming AVJ, distinguished by thegrooming time slots, which must be passed back from the original connectmessage. In a point-to-point connection using for example SS7 signaling,these circled pieces of information are not necessary because any timeslot P to any other time slot Q has just one unique connection. Adisconnect ion can be facilitated by a bare minimum of information ofjust one end of the connection, for example timeslot Q, and both are notnecessary to be known as described herein. The notepad area is used forannotation as the worm is passed forward, for example to hold the offsetinto the trunk group between each successive AVJ stage. It is distinctfrom the annotated information uniquely identifying the voice path suchas the choice of a grooming time slot. The modify command is associatedwith further subsequent inclusion or exclusion of echo cancellers in thevoice path, which is just one specific example of path modification, alltypes of modification are envisaged, in this particular case it may bedesirable to move a grooming time slot away from a half echo cancellerdevice to a grooming time slot which is a simple loop-back function orvice versa, to free up resources by inverting the original echocanceller indication with an optional facility to specify the groomingtime slot and the worm could be subsequently annotated with the new timeslot, specifying at minimum, the thinking time slot, the grooming AVJidentity, the indication of half echo canceller, and forpoint-to-multipoint connections above the grooming time slots and theVTG.

FIG. 19 shows the way in which a voice path is traced through the AGRnetwork system. The trunking time slot is common to both the ingress andthe egress process for a both-way circuit, and is readily available inthe TDM domain. Hence the trunking time slot for both ingress and egressprocesses with respect to the ATM is known such that, at step 1, thetrunking timeslot location in the egress connection map contains aparticular VCJ identity and the connection entity for that VCJ theoffset of that trunking time slot in the VCJ trunk group, Trunking TimeSlot (T.TS) in the upstream direction. That offset can be passed to thenext AVJ stage, the grooming AVJ. Since VCJs are configured asreciprocating pairs, at step 2, knowing the upstream VCJ also meansknowing the downstream VCJ. The ingress process can then, from the VCJconnection map, determine the offset of the trunking time slot T.T.S(step 3) in that particular VCJ, the offset downstream, offset D. (step4) The offset of the upstream and the downstream direction in thetrunking AVJ have now been determined and can be passed by signaling asdescribed above to the grooming AVJ in the next stage of the connection.At the grooming AVJ, from the offset in the upstream direction in thegiven VCJ, the ingress process at step 2 can determine the groomingtimeslot in the upstream direction (step 3) which is the timeslot usedin the upstream direction on the VCJ side and on the VTG side. From thegrooming timeslot in the upstream direction the egress process candetermine the VTG from the contents of the location in the map and in anidentical manner to the trunking AVJ stage, can determine the offset ofthat grooming timeslot in the VTG trunk group, the offset in theupstream direction (step 3). From the offset in the downstream theegress process can determine the particular grooming timeslot in thedownstream trunk group that occurs at that offset (step 5) andconsequently from complementary pair of the upstream VTG, it determinesthe downstream VTG at step 4. The ingress process can thus determine theoffset in the downstream direction of that grooming timeslot in thatparticular downstream VTG also at step 5, and thus the offset in thedownstream direction and the offset in the upstream direction can besignaled to the next grooming or trunking stage, and the process cancontinue in a cascade in a similar manner so as to determine thecomplete voice path through to the trunk circuit P at the remote end.This provides resilience of the system, for as long as the hardware isreliable. Even where the software could be largely out-of-date, it couldalways recover the hardware state and determine what voice paths areestablished in the network, and resume full operation subsequently.

FIG. 20 shows the overview of the whole wormhole protocol. Step 1 showsthe ITU SS7 message, i.e. the IAM message identifying trunk circuit P.This also contains dialed digits which call routing processes,determines the voice route by consideration for external congestion anda prioritized set of external voice routes (step 2) which it passes toconnection control at step 3. Connection control checks the internalcongestion from the VTR capacity monitors and, if necessary, (step 4),connection control may also initiate any SS7 loop back continuity checkson trunk circuit P or any other type of continuity check appropriate tothe signaling system employed. Connection control eliminates thoseroutes which are unreachable through internal congestion and putsforward its preferred route for load balancing where there is an equalexternal voice route. Call routing determines the final choice of voiceroute and forwards the message. At step 5 the outgoing call routingselects the outgoing trunk circuit and resolves any glare (contentionfor trunk circuits that may be coming in from its own incoming processon this AGR). Once the circuit is selected, connection control checksinternal congestion again through the VTR capacity monitors and makes afinal selection of the VTG that connects the associated pair of AGRs andcreates a worm which comprises sufficient topological information toconnect trunk circuit Q to trunk circuit P via the chosen VTG andforwards this at step 7 to the connection entity inside the trunking AVJhosting Q, which may also initiate a continuity check or any otherscheme provided by the signaling system on trunk circuit Q step 8. Theworm is passed forward through the grooming AVJ stages which mayoptionally include or exclude half echo cancellers in this pathinitially, and they may be excluded at any later stage by a modifiedworm. The grooming AVJ forwards the worm through the VTG to the remoteAGR on the incoming side (step 9) which connects the path through totrunk circuit P and also then passes the worm back in the reversedirection (step 10) to set up in the reverse direction. As a form ofacknowledgement, the annotated information as to what the path was inthe worm information is stored in connection control at both ends sothat the path can be torn down from either end using any of the existingschemes in the signaling system. Once the worm has passed back to theoutgoing side it is passed back to connection control and call routingwhich will then signal an outgoing IAM message (step 11) so as to enablethrough-connections which are shown as switches in FIG. 20. It thusfacilitates the connection of a voice path through this single largedistributed exchange system. Step 12 is an optional stage to check theintegrity of the voice path to enable the through connection and theoutgoing IAM message.

The arrangement and method described provide for the distribution of theexchange in terms of its fabric and its control and its enablingtechnologies with reference to the dynamic structure data transfer.There is uncommitted bandwidth within the fabric and consequentlyrouting decisions can be fully independent of the operation of thisdistributed fabric. The distributed fabric can, because of thisunallocated bandwidth use separate connections to the control layer forestablishing the connections through the fabric and this in no waycompromises external decisions that are made except when an overloadsituation is encountered. An advertising process provides knowledge ofthe distributed fabric. This includes local knowledge about a remotesite such that routing decisions can be made and modified wherenecessary so as to reject traffic at source. The dynamic trunkingenables the separation of call routing and connection control. Thearrangement also provides a means of ensuring stability under overloadsituations and minimizing the cost of handling traffic which would berejected by destinations.

The separation of call routing and connection control together withadvertising the system status ensures a wide range of scalability sothat the application of dynamic trunking technology provides scalabilityin a traffic sense. Further, the separation of call routing andconnection control provides a distributed computing environment which isscalable and managed by this advertisement resource. Because thedistributed exchange manages its own internal traffic, effectively itprovides means for balancing that traffic to the fabric and makes itsown internal routing decisions. Once that network grows through itsscalability, a local exchange can dual home on to it and allow it tomake its own routing decisions. The local exchange does not have to makeany individual routing decisions to a variety of traffic exchanges. Theseparation further provides a facility to support a wide range ofservices using other signaling schemes. The fabric provides a connectionengine that can accommodate a wide variety of signaling protocolsappropriate for the type of service to be provided and that can set upconnections.

The full knowledge of the network connectivity and the release ofresources can be enabled by any node within the network, as it can tracethrough all connections from any starting point. This can be used tosupport failure recovery.

It will be appreciated that although the arrangement and method havebeen described above with particular reference to current standardprotocols, such as SS7 signaling, it is in no way limited to the use ofthese particular protocols.

What is claimed is:
 1. A telecommunications system comprising: abroadband digital communications network; a first exchange connected tothe broadband digital communications network at a plurality of networkingress points; one or more monitors configured to monitor internalcongestion in at least part of the broadband digital communicationsnetwork; wherein said internal congestion comprises at least calltraffic competing for available resources; and a load balancerconfigured to select one of said ingress points through which tocommunicate call traffic from said first exchange into the broadbanddigital communications network, said selecting being in response to saidmonitored congestion.
 2. A telecommunications system according to claim1, wherein the ingress point is selected to balance load across thebroadband digital communications network.
 3. A telecommunications systemaccording to claim 1, wherein the ingress point is selected to balanceload on routes connected to the broadband digital communicationsnetwork.
 4. A telecommunications system according to claim 1comprising:— a second exchange connected to the broadband digitalcommunications network at a plurality of network egress points; whereinthe load balancer is configured to select one of the egress pointsthrough which to route said call traffic out of the broadband digitalcommunications network, in response to said monitored congestion.
 5. Atelecommunications system according to claim 1, wherein the broadbanddigital communications network comprises a plurality of nodes not beingingress or egress points for said call traffic, and wherein the loadbalancer is configured to select one of said nodes through which toroute said call traffic, in response to said monitored congestion.
 6. Atelecommunications system according to claim 1, wherein the one or moremonitors are arranged to monitor the capacity of one or more virtualtrunks interconnecting nodes of the broadband digital communicationsnetwork.
 7. A telecommunications system according to claim 1, whereinthe one or more monitors are distributed across one or more nodes of thebroadband digital communications network.
 8. A telecommunications systemaccording to claim 1, wherein the one or more monitors are arranged toadvertise congestion status from one or more local to one or more remotenodes of the broadband digital communications network.
 9. Atelecommunications system according to claim 8, wherein the one or moremonitors are arranged to advertise congestion status to only a selectionof nodes of the broadband digital communications network.
 10. Atelecommunications system according to claim 9, wherein the selection ofnodes is on the basis of geographical distance from a local node.
 11. Atelecommunications system according to claim 9, wherein the selection ofnodes is on the basis of logical placement with respect to a local node.12. A telecommunications system according to claim 1, wherein thebroadband digital communications network is a packet-switched network.13. A telecommunications system according to claim 1, wherein thebroadband digital communications network is a connection-orientednetwork.
 14. A telecommunications system according to claim 13, whereinthe broadband digital communications network is an asynchronous transfermode (ATM) network.
 15. A telecommunications system according to claim1, wherein the broadband digital communications network is a wide areanetwork (WAN).
 16. A telecommunications system according to claim 1,wherein the broadband digital communications network carries types oftraffic other than said call traffic.
 17. A telecommunications systemcomprising: a broadband digital communications network; means connectedto the broadband digital communications network at a plurality ofnetwork ingress points and configured to communicate traffic to thebroadband digital communications network; means for monitoring internalcongestion in at least part of the broadband digital communicationsnetwork; wherein said internal congestion comprises at least calltraffic competing for available resources; and means for selecting oneof said ingress points through which to communicate call traffic fromsaid first exchange into the broadband digital communications network,said selecting being in response to said monitored congestion.
 18. Atelecommunications system comprising: a broadband digital communicationsnetwork; a second exchange connected to the broadband digitalcommunications network at a plurality of network egress points; one ormore monitors configured to monitor internal congestion in at least partof the broadband digital communications network; wherein said internalcongestion comprises at least call traffic competing for availableresources; and a load balancer configured to select one of said egresspoints through which to communicate call traffic from the broadbanddigital communications network to the second exchange, said selectingbeing in response to said monitored congestion.
 19. A telecommunicationssystem according to claim 18, wherein the egress point is selected tobalance load across the broadband digital communications network.
 20. Atelecommunications system according to claim 18, wherein the egresspoint is selected to balance load on routes connected to the broadbanddigital communications network.
 21. A telecommunications systemaccording to claim 18, further comprising:— a first exchange connectedto the broadband digital communications network at a plurality ofnetwork ingress points; wherein the load balancer is configured toselect one of the ingress points through which to route said calltraffic into the broadband digital communications network, in responseto said monitored congestion.
 22. A telecommunications system accordingto claim 18, wherein the broadband digital communications networkcomprises a plurality of nodes not being ingress or egress points forsaid call traffic, and wherein the load balancer is configured to selectone of said nodes through which to route said call traffic, in responseto said monitored congestion.
 23. A telecommunications system accordingto claim 18, wherein the one or more monitors are arranged to monitorthe capacity of one or more virtual trunks interconnecting nodes of thebroadband digital communications network.
 24. A telecommunicationssystem according to claim 18, wherein the one or more monitors aredistributed across one or more nodes of the broadband digitalcommunications network.
 25. A telecommunications system according toclaim 18, wherein the one or more monitors are arranged to advertisecongestion status from one or more local to one or more remote nodes ofthe broadband digital communications network.
 26. A telecommunicationssystem according to claim 25, wherein the one or more monitors arearranged to advertise congestion status to only a selection of nodes ofthe broadband digital communications network.
 27. A telecommunicationssystem according to claim 26, wherein the selection of nodes is on thebasis of geographical distance from a local node.
 28. Atelecommunications system according to claim 26, wherein the selectionof nodes is on the basis of logical placement with respect to a localnode.
 29. A telecommunications system according to claim 18, wherein thebroadband digital communications network is a packet-switched network.30. A telecommunications system according to claim 18, wherein thebroadband digital communications network is a connection-orientednetwork.
 31. A telecommunications system according to claim 30, whereinthe broadband digital communications network is an asynchronous transfermode (ATM) network.
 32. A telecommunications system according to claim18, wherein the broadband digital communications network is a wide areanetwork (WAN).
 33. A telecommunications system according to claim 18,wherein the broadband digital communications network carries types oftraffic other than said call traffic.
 34. A telecommunications systemcomprising: a broadband digital communications network; means connectedto the broadband digital communications network at a plurality ofnetwork egress points and configured to communicate traffic from thebroadband digital communications network; means for monitoring internalcongestion in at least part of the broadband digital communicationsnetwork; wherein said internal congestion comprises at least calltraffic competing for available resources; and means for selecting oneof said egress points through which to communicate call traffic from thebroadband digital communications network to the means connected to thenetwork, said selecting being in response to said monitored congestion.35. A node for load balancing call traffic in a telecommunicationssystem comprising a first exchange connected to a broadband digitalcommunications network at a plurality of network ingress points, thenode comprising: a selector configured to select one of said ingresspoints through which to communicate call traffic from said firstexchange into the broadband digital communications network, theselecting being in response to an assessment of internal congestion inat least part of the broadband digital communications network; whereinsaid internal congestion comprises at least call traffic competing foravailable resources.
 36. A node according to claim 35, wherein theingress point is selected to balance load across the broadband digitalcommunications network.
 37. A node according to claim 35, wherein theingress point is selected to balance load on routes connected to thebroadband digital communications network.
 38. A node according to claim35, wherein a second exchange is connected to the broadband digitalcommunications network at a plurality of network egress points, the nodebeing arranged to select one of the egress points through which to routesaid call traffic out of the broadband digital communications network,in response to an assessment of congestion in the telecommunicationssystem.
 39. A node according to claim 35, wherein the network comprisesa plurality of nodes not being ingress or egress points for said calltraffic, the node being arranged to select one of said plurality ofnodes through which to route said call traffic, in response to anassessment of congestion in the broadband digital communicationsnetwork.
 40. A node according to claim 35, wherein the broadband digitalcommunications network is a packet-switched network.
 41. A nodeaccording to claim 35, wherein the broadband digital communicationsnetwork is a connection-oriented network.
 42. A node according to claim41, wherein the broadband digital communications network is anasynchronous transfer mode (ATM) network.
 43. A node according to claim35, wherein the broadband digital communications network is a wide areanetwork (WAN).
 44. A node according to claim 35, wherein the broadbanddigital communications network carries types of traffic other than saidcall traffic.
 45. A node for load balancing call traffic in atelecommunications system comprising means connected to a broadbanddigital communications network at a plurality of network ingress pointsand configured to communicate traffic to the broadband digitalcommunications network, the node comprising: a processor to select oneof said ingress points through which to communicate call traffic fromsaid means connected to the network, the selecting being in response toan assessment of internal congestion in at least part of the broadbanddigital communications network; wherein said internal congestioncomprises at least call traffic competing for available resources.
 46. Anode for load balancing call traffic in a telecommunications systemcomprising a second exchange connected to a broadband digitalcommunications network at a plurality of network egress points, the nodecomprising: a selector configured to select one of said egress pointsthrough which to communicate call traffic from the broadband digitalcommunications network to said second exchange, the selecting being inresponse to an assessment of internal congestion in at least part of thebroadband digital communications network; wherein said internalcongestion comprises at least call traffic competing for availableresources.
 47. A node according to claim 46, wherein the egress point isselected to balance load across the broadband digital communicationsnetwork.
 48. A node according to claim 46, wherein the egress point isselected to balance load on routes connected to the broadband digitalcommunications network.
 49. A node according to claim 46, wherein afirst exchange is connected to the broadband digital communicationsnetwork at a plurality of network ingress points, the node beingarranged select one of the ingress points through which to route saidcall traffic into the broadband digital communications network, inresponse to an assessment of congestion in the broadband digitalcommunications network.
 50. A node according to claim 46, wherein thebroadband digital communications network comprises a plurality of nodesnot being ingress or egress points for said call traffic, the node beingarranged to select one of said plurality of nodes through which to routesaid call traffic, in response to an assessment of congestion in thebroadband digital communications network.
 51. A node according to claim46, wherein the broadband digital communications network is apacket-switched network.
 52. A node according to claim 46, wherein thebroadband digital communications network is a connection-orientednetwork.
 53. A node according to claim 52, wherein the broadband digitalcommunications network is an asynchronous transfer mode (ATM) network.54. A node according to claim 46, wherein the broadband digitalcommunications network is a wide area network (WAN).
 55. A nodeaccording to claim 46, wherein the broadband digital communicationsnetwork carries types of traffic other than said call traffic.
 56. Anode for load balancing call traffic in a telecommunications systemcomprising means connected to a broadband digital communications networkat a plurality of network egress points and configured to communicatetraffic from the broadband digital communications network, the nodecomprising: a processor for selecting one of said egress points throughwhich to communicate call traffic from the broadband digitalcommunications network to said means connected to the broadband digitalcommunications network, the selecting being in response to an assessmentof internal congestion in at least part of the broadband digitalcommunications network; wherein said internal congestion comprises atleast call traffic competing for available resources.
 57. A method ofload balancing call traffic in a telecommunications system comprising afirst exchange connected to a broadband digital communications networkat a plurality of network ingress points, the method comprising thefollowing steps: monitoring internal congestion in at least part of thebroadband digital communications network; wherein said internalcongestion comprises at least call traffic competing for availableresources; and selecting one of said ingress points through which tocommunicate call traffic from said first exchange into the broadbanddigital communications network in response to said monitoring.
 58. Amethod according to claim 57, wherein the ingress point is selected tobalance load across the broadband digital communications network.
 59. Amethod according to claim 57, wherein the ingress point is selected tobalance load on routes connected to the broadband digital communicationsnetwork.
 60. A method according to claim 57, wherein a second exchangeis connected to the broadband digital communications network at aplurality of network egress points, and wherein the method furthercomprises the step of: selecting an egress point through which to routesaid call traffic out of the broadband digital communications network,in response to said monitoring.
 61. A method according to claim 57,wherein the broadband digital communications network comprises aplurality of nodes not being ingress or egress points for said calltraffic, and wherein the method further comprises the step of: selectingone of said plurality of nodes through which to route said call trafficin response to said monitoring.
 62. A method according to claim 57,wherein the monitoring comprises monitoring the capacity of one or morevirtual trunks interconnecting nodes of the broadband digitalcommunications network.
 63. A method according to claim 57, whereinmonitoring congestion is performed at one or more nodes of the broadbanddigital communications network.
 64. A method according to claim 57,further comprising advertising congestion status from one or more localto one or more remote nodes of the broadband digital communicationsnetwork.
 65. A method according to claim 64, wherein the advertisingcongestion status is to only a selection of nodes of the broadbanddigital communications network.
 66. A method according to claim 65,wherein the selection of nodes is on the basis of geographical distancefrom a local node.
 67. A method according to claim 65, wherein theselection of nodes is on the basis of logical placement with respect toa local node.
 68. A method according to claim 57, wherein the broadbanddigital communications network is a packet-switched network.
 69. Amethod according to claim 57, wherein the broadband digitalcommunications network is a connection-oriented network.
 70. A methodaccording to claim 69, wherein the broadband digital communicationsnetwork is an asynchronous transfer mode (ATM) network.
 71. A methodaccording to claim 57, wherein the broadband digital communicationsnetwork is a wide area network (WAN).
 72. A method according to claim57, wherein the broadband digital communications network carries typesof traffic other than said call traffic.
 73. A method of load balancingcall traffic in a telecommunications system comprising a second exchangeconnected to a broadband digital communications network at a pluralityof network egress points, the method comprising the following steps:monitoring internal congestion in at least part of the broadband digitalcommunications network; wherein said internal congestion comprises atleast call traffic competing for available resources; selecting one ofsaid egress points through which to communicate call traffic from saidbroadband digital communications network to said second exchange inresponse to said monitoring.
 74. A method according to claim 73, whereinthe egress point is selected to balance load across the broadbanddigital communications network.
 75. A method according to claim 73,wherein the egress point is selected to balance load on routes connectedto the broadband digital communications network.
 76. A method accordingto claim 73, wherein a first exchange is connected to the broadbanddigital communications network at a plurality of network ingress points;and wherein the method further comprises the step of: selecting aningress point through which to route said call traffic into thebroadband digital communications network, in response to saidmonitoring.
 77. A method according to claim 73, wherein the broadbanddigital communications network comprises a plurality of nodes not beingingress or egress points for said call traffic, and wherein the methodfurther comprises the step of: selecting one of said plurality of nodesthrough which to route said call traffic in response to said monitoring.78. A telecommunications system, comprising: a broadband digitalcommunications network; one or more monitors configured to monitorresource availability in at least part of the broadband digitalcommunications network; and a call handler configured to handle incomingcalls to the broadband digital communications network in dependence onsaid resource availability, thereby to increase the likelihood ofsuccessfully connecting said calls across the broadband digitalcommunications network, wherein said monitored resource availabilitycomprises and assessment of internal congestion on a plurality ofpotential call routes through the broadband digital communicationsnetwork, and wherein the telecommunications system is arranged to selectbetween said plurality of potential call routes for routing an incomingcall according to said assessment; wherein said assessment of internalcongestion comprises an assessment of at least call traffic competingfor available resources.
 79. A telecommunications system according toclaim 78, wherein said monitored resource availability comprises anindication of at least part of said system remote from an ingress pointthrough which call traffic of an incoming call may be routed into thebroadband digital communications network.
 80. A telecommunicationssystem according to claim 78, wherein said monitored resourceavailability comprises availability of a call destination point for anincoming call and wherein the telecommunications system is arranged toreject the call at source in the event that the destination point isunavailable.
 81. A telecommunications system according to claim 78,further comprising: an exchange connected to the broadband digitalcommunications network at a plurality of network ingress points; whereinsaid monitored resource availability comprises an assessment ofcongestion in the telecommunications system and wherein one of saidingress points through which to route call traffic from said exchangeinto the broadband digital communications network is selected independence on said assessment.
 82. A telecommunications system accordingto claim 78, wherein the one or more monitors are arranged to monitorthe capacity of one or more virtual trunks interconnecting nodes of thebroadband digital communications network.
 83. A telecommunicationssystem according to claim 78, wherein the one or more monitors aredistributed across one or more nodes of the broadband digitalcommunications network.
 84. A telecommunications system according toclaim 78, wherein the one or more monitors are arranged to advertisecongestion status from one or more local to one or more remote nodes ofthe broadband digital communications network.
 85. A telecommunicationssystem according to claim 84, wherein the one or more monitors arearranged to advertise congestion status to only a selection of nodes ofthe broadband digital communications network.
 86. A telecommunicationssystem according to claim 85, wherein the selection of nodes is on thebasis of geographical distance from a local node.
 87. Atelecommunications system according to claim 84, wherein the selectionof nodes is on the basis of logical placement with respect to a localnode.
 88. A telecommunications system according to claim 78, wherein thebroadband digital communications network is a packet-switched network.89. A telecommunications system according to claim 78, wherein thebroadband digital communications network is a connection-orientednetwork.
 90. A telecommunications system according to claim 89, whereinthe broadband digital communications network is an asynchronous transfermode (ATM) network.
 91. A telecommunications system according to claim78, wherein the broadband digital communications network is a wide areanetwork (WAN).
 92. A telecommunications system according to claim 78,wherein the broadband digital communications network carries types oftraffic other than said call traffic.
 93. A call routing server for usein a telecommunications system having a broadband digital communicationsnetwork and one or more monitors for monitoring resource availability inat least part of the broadband digital communications network, the callrouting server comprising: a call handler configured to handle incomingcalls in dependence on said monitored resource availability in thebroadband digital communications network, thereby to increase thelikelihood of successfully connecting said calls across the broadbanddigital communications network; said monitored resource availabilitycomprises an assessment of internal congestion on a plurality ofpotential call routes, and wherein the call routing server is arrangedto select between said plurality of potential call routes for routing anincoming call according to said assessment; wherein said assessment ofinternal congestion comprises an assessment of at least call trafficcompeting for available resources.
 94. A call routing server accordingto claim 93, wherein said monitored resource availability comprisesavailability of a call destination point for an incoming call andwherein the call routing server is arranged to reject the call at sourcein the event that the destination point is unavailable.
 95. A callrouting server according to claim 93, wherein said monitored resourceavailability comprises an assessment of congestion in the broadbanddigital communications network and wherein the telecommunications systemis arranged to reject at source an incoming call to be routed through aspecified endpoint of the network if said endpoint is unreachable as aresult of said assessment.
 96. A call routing server according to claim93, wherein the broadband digital communications network is apacket-switched network.
 97. A call routing server according to claim93, wherein the broadband digital communications network is aconnection-oriented network.
 98. A call routing server according toclaim 97, wherein the broadband digital communications network is anasynchronous transfer mode (ATM) network.
 99. A call routing serveraccording to claim 93, wherein the broadband digital communicationsnetwork is a wide area network (WAN).
 100. A call routing serveraccording to claim 93, wherein the broadband digital communicationsnetwork carries types of traffic other than said call traffic.
 101. Amethod of handling incoming calls to be routed across a broadbanddigital communications network, the method comprising the steps of:monitoring resource availability in at least part of atelecommunications system comprising said broadband digitalcommunications network; and handling an incoming call in dependence onsaid resource availability, thereby to increase the likelihood ofsuccessfully connecting the call across the broadband digitalcommunications network, wherein said monitored resource availabilitycomprises an assessment of internal congestion on a plurality ofpotential call routes, and wherein the method comprises the step ofselecting between said plurality of potential call routes for routingsaid call according to said assessment; wherein said assessment ofinternal congestion comprises an assessment of at least call trafficcompeting for available resources.
 102. A method according to claim 101,wherein said monitored resource availability comprises availability of acall destination point for an incoming call and wherein the methodfurther comprises the step of rejecting an incoming call at source inthe event that the destination point is unavailable.
 103. A methodaccording to claim 101, wherein said monitored resource availabilitycomprises an assessment of congestion in the broadband digitalcommunications network and wherein the method further comprises the stepof rejecting at source an incoming call to be routed through a specifiedendpoint of the broadband digital communications network if saidendpoint is unreachable as a result of said assessment.
 104. A methodaccording to claim 101, wherein the telecommunications system furthercomprises an exchange multi homed to the broadband digitalcommunications network at a plurality of network ingress points, whereinsaid monitored resource availability comprises an assessment ofcongestion in the broadband digital communications network, and whereinthe method further comprises the step of selecting one of said ingresspoints through which to route call traffic from said exchange into thebroadband digital communications network in dependence on saidassessment.
 105. A method according to claim 101, wherein the step ofmonitoring resource availability comprises monitoring the capacity ofone or more virtual trunks interconnecting nodes of the broadbanddigital communications network.
 106. A method according to claim 101,wherein the step of monitoring resource availability comprisesadvertising congestion status from one or more local to one or moreremote nodes of the broadband digital communications network.
 107. Amethod according to claim 106, wherein congestion status is onlyadvertised to a selection of nodes of the broadband digitalcommunications network.
 108. A method according to claim 107, whereinthe selection of nodes is on the basis of geographical distance from alocal node.
 109. A method according to claim 107, wherein the selectionof nodes is on the basis of logical placement with respect to a localnode.
 110. A method according to claim 101, wherein the broadbanddigital communications network is a packet-switched network.
 111. Amethod according to claim 101, wherein the broadband digitalcommunications network is a connection-oriented network.
 112. A methodaccording to claim 111, wherein the broadband digital communicationsnetwork is an asynchronous transfer mode (ATM) network.
 113. A methodaccording to claim 101, wherein the broadband digital communicationsnetwork is a wide area network (WAN).
 114. A method according to claim101, wherein the broadband digital communications network carries typesof traffic other than said call traffic.
 115. A telecommunicationssystem, comprising: a broadband digital communications network includinga plurality of nodes; an exchange connected to the broadband digitalcommunications network at a plurality of points; first and second callrouting servers, the first call routing server being configured to:select a call route through which call traffic may be transmitted, thecall route identifying the point through which to communicate calltraffic between the exchange and the broadband digital communicationsnetwork, and communicate said call route using one or more narrowbandsignaling protocol messages to the second call routing server, saidsignaling messages being in respect of a potential call across thenetwork and being passed through nodes of the network without furtherprocessing; wherein said call route is selected based on an assessmentof internal congestion; wherein said assessment of internal congestioncomprises an assessment of at least call traffic competing for availableresources.
 116. A telecommunications system according to claim 115,wherein a call route through which traffic may be transmitted identifiesendpoints of the broadband digital communications network between whichcall traffic may be transmitted.
 117. A telecommunications systemaccording to claim 115, wherein the narrowband signalling protocol isN-ISUP.
 118. A telecommunications system according to claim 115, whereinthe first call routing server is distributed across a plurality of nodesof the broadband digital communications network.
 119. Atelecommunications system according to claim 115, wherein the first callrouting server is a proxy call routing server.
 120. A telecommunicationssystem according to claim 119, wherein the proxy call routing serverimplements a service that goes beyond the routing capabilities of thenarrowband signalling protocol.
 121. A telecommunications systemaccording to claim 119, wherein the proxy call routing server is amultimedia server.
 122. A telecommunications system according to claim115, wherein the broadband digital communications network is apacket-switched network.
 123. A telecommunications system according toclaim 115, wherein the broadband digital communications network is aconnection-oriented network.
 124. A telecommunications system accordingto claim 123, wherein the broadband digital communications network is anasynchronous transfer mode (ATM) network.
 125. A telecommunicationssystem according to claim 115, wherein the broadband digitalcommunications network is a wide area network (WAN).
 126. A first callrouting server for use in a telecommunications system comprising abroadband digital telecommunications network, a connection controllerfor establishing, maintaining or removing connections in the network fortransmitting call traffic, and exchange connected to the broadbanddigital communications network at a plurality of points, and a secondcall routing server, the first call routing server comprising: aprocessor cofigured to select a call route through which call trafficmay be transmitted, the call route identifying the point through whichto communicate call traffic between the exchange and the broadbanddigital communications network, and an output to forward said call routeto the second call routing server using narrowband signaling protocolmessages, said signaling messages being passed through nodes of thebroadband communications network without further processing, thereby tocause the connection controller to establish, modify or removeconnections in the broadband digital communications network fortransmitting said call traffic; wherein said call route is selectedbased on an assessment of internal congestion; wherein said assessmentof internal congestion comprises an assessment of at least call trafficcompeting for available resources.
 127. A first call routing serveraccording to claim 126, wherein the call route identifies endpoints ofthe broadband digital communications network between which call trafficmay be transmitted.
 128. A first call routing server according to claim126, wherein the narrowband signalling protocol is N-ISUP.
 129. A firstcall routing server according to claim 126, wherein the first callrouting server is a proxy call routing server.
 130. A first call routingserver according to claim 129, wherein the proxy call routing serverimplements a service that goes beyond the routing capabilities of thenarrowband signalling protocol.
 131. A first call routing serveraccording to claim 129, wherein the proxy call routing server is amultimedia server.
 132. A first call routing server according to claim126, wherein the broadband digital communications network is apacket-switched network.
 133. A first call routing server according toclaim 126, wherein the broadband digital communications network is aconnection-oriented network.
 134. A first call routing server accordingto claim 133, wherein the broadband digital communications network is anasynchronous transfer mode (ATM) network.
 135. A first call routingserver according to claim 126, wherein the broadband digitalcommunications network is a wide area network (WAN).
 136. A method ofestablishing a connection for transmitting call traffic over a broadbanddigital communications network in a telecommunications system, thetelecommunications system comprising an exchange connected to thebroadband digital communications network at a plurality of points, themethod comprising the steps of: a first call routing server selecting acall route through which call traffic may be transmitted, the call routeidentifying the point through which to communicate call traffic betweenthe exchange and the broadband digital communications network, the firstcall routing server forwarding said call route to a second call routingserver using narrowband signaling protocol messages, said signalingmessages being passed through nodes of the network without furtherprocessing, in response to said forwarding, establishing a connectionfor transmitting call traffic over the network; wherein said call routeis selected based on an assessment of internal congestion; wherein saidassessment of internal congestion comprises an assessment of at leastcall traffic competing for available resources.
 137. A method accordingto claim 136, wherein the call route identifies endpoints of thebroadband digital communications network between which call traffic maybe transmitted.
 138. A method according to claim 136, wherein thenarrowband signalling protocol is N-ISUP.
 139. A method according toclaim 136, wherein the broadband digital communications network is apacket-switched network.
 140. A method according to claim 136, whereinthe broadband digital communications network is a connection-orientednetwork.
 141. A method according to claim 140, wherein the broadbanddigital communications network is an asynchronous transfer mode (ATM)network.
 142. A method according to claim 136, wherein the broadbanddigital communications network is a wide area network (WAN).
 143. Atelecommunications system, comprising: a first communications networkincluding a plurality of nodes; an exchange connected to the firstcommunications network at a plurality of points; a first call routingserver communicating one or more signaling protocol messages originatingin a second communications network to a second call routing server, saidsignaling messages being in respect of a call route for a potential callacross the first communications network, the call route identifying thepoint through which to communicate call traffic between the exchange andthe first communications network; and passing said one or more signalingprotocol messages through nodes of the first communications networkwithout further processing, wherein the first and second communicationsnetwork use different signaling protocols; wherein said call route isselected based on an assessment of internal congestion; wherein saidassessment of internal congestion comprises an assessment of at leastcall traffic competing for available resources.
 144. Atelecommunications system, comprising: a first communications networkincluding a plurality of nodes; and first and second call routingservers, the first call routing server being configured to communicateone or more signaling protocol messages originating in a secondcommunications network to the second call routing server, said signalingmessages being in respect of a call route for a potential call acrossthe first communications network, the call route identifying a pointthrough which to communicate call traffic between an exchange and thefirst communications network, and being passed through nodes of thefirst communications network without further processing, wherein thefirst and second communications networks use different signalingprotocols and the exchange is connected to the first communicationsnetwork at a plurality of points; wherein said call route is selectedbased on an assessment of internal congestion; wherein said assessmentof internal congestion comprises an assessment of at least call trafficcompeting for available resources.